The long-term goal of this project is to determine the effects of Cryptosporidium parvum infection and the presence of intestinal microflora on the development of inflammatory bowel disease (IBD) - like lesions in immunocompromised mice. We will utilize germfree and conventional alpha/beta T cell receptor(TCR)-deficient mice as a model in order to examine the role of B-1 (Ly-1; CD5+) B cells in the development of intestinal inflammatory lesions in the absence of alpha beta T cells. Specifically. it is hypothesized that, in the absence of regulatory alpha/beta T cells, overt activation of B-1 (Ly-I) B cells in response to antigens of intestinal microflora contribute to the development of IBD-like lesions in flora-bearing mice infected with C. parvum. Moreover it is postulated that in germfree alpha/beta deficient mice IBD-like lesions will not develop. We will test this hypothesis by comparing the development of IBD in both germfree and conventional alpha/beta T cell- deficient mice. The lesions will be characterized histologically and immunohistochemically to assess the nature of the host response. We will also examine the effect of selectively colonizing germfree alpha/beta T cell-deficient mice with defined bacteria isolated from the normal murine intestinal flora on the outcome of C. parvum infection and development of IBD lesions. The effect of C. parvum infection upon the activation of B-1 B cells will be assessed in the presence and absence of intestinal microorganisms. As a result of preliminary studies, the kinetics of the onset of IBD in conventional alpha/beta T cell-deficient mice infected with C. parvum have already been determined and it was observed that C. parvum infection greatly accelerates the onset of IBD, This information will be used to examine immunologic parameter's at specific times during the progression of disease. An additional strength of this proposed approach is the ability to determine cellular responses associated with the onset of colitis, studies which are difficult to accomplish in studies of humans with IBD or in rodent models employing more long term development of spontaneous colitis. The proposed research provides a rapid and reproducible model to study the development of IBD-like lesions and affords the opportunity to determine the contribution of normal intestinal flora to the process of the disease. In addition, this model will further our understanding of the pathogenesis of cryptosporidiosis in the setting of the immunocompromised host.